Download the appropriate system image for your device below, then unzip it to a safe directory.
Connect your device to your computer over USB.
Start the device in fastboot mode with one of the following methods:
Using the adb tool: With the device powered on, execute:
Using a key combo: Turn the device off, then turn it on and immediately hold down the relevant key combination for your device.
If necessary, unlock the devices bootloader using one of the following methods:

If you are updating a Nexus or Pixel device that is manufactured in 2015 or later (for example, a Nexus 5X, Nexus 6P, Pixel, Pixel XL, Pixel 2 or Pixel 2 XL device), run this command:
For Pixel 2: To flash the bootloader, Pixel 2s boot loader must be updated to at least Oreo MR1s version first. This may be done by applying an over-the-air (OTA) update, or sideloading a full OTA with the instructions on that page.
For Pixel 2 XL only with loader version prior to TMZ20a: the critical partitions may also need to be unlocked before flashing. The unlock can be performed with this command, and should NOT be done on other devices:
If you are updating an older device, run this command:
The target device will show you a confirmation screen. (This erases all data on the target device.)
See Unlocking the bootloader for more detailed instructions.
Open a terminal and navigate to the unzipped system image directory.